1. Field of Disclosure
Aspects of the present invention relate to an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses using an electrophotographic method such as a copying machine and a printer, conventionally employ contact charging devices because of advantages such as low ozone emission and low power. Contact charging devices charge a photosensitive drum by applying a voltage to a charging member in contact with the photosensitive drum. In particular, contact charging devices of a roller charging method, using a charging roller as a charging member, are preferred in terms of charging stability and are in widespread use. With a contact charging device of the roller charging method, the surface potential of a photosensitive drum starts to increase when a voltage higher than or equal to a certain level (charging start voltage Vth) is applied to the charging roller. The surface potential of the photosensitive drum thereafter increases linearly with a gradient of one with respect to the applied voltage. To obtain a photosensitive drum surface potential (Vd) necessary for electrophotography, a direct-current (DC) voltage of Vd+Vth needs to be applied to the charging member.
As a method for improving uniformity of the surface potential of the photosensitive drum in a DC charging system, the following method has been discussed (see Japanese Patent Application Laid-Open No. 8-171260). A primary charging device once charges the photosensitive drum to a potential higher than or equal to a non-image portion potential (Vd) necessary for image formation. An exposure unit (post-exposure unit) arranged in a position after the primary charging and before development emits weak light to expose the potential of the photosensitive drum, thereby attenuating (lowering) the surface potential. A target non-image portion potential (Vd) can be obtained by such a potential control method.
Using the DC charging system, the charging start voltage Vth varies depending on the thickness of a photosensitive layer of the photosensitive drum. As the photosensitive drum is shaven, the thickness of the photosensitive drum is reduced and the non-image portion potential (Vd) increases. A method for calculating the thickness of a photosensitive drum from information about any of the number of passed sheets, the number of rotations of the photosensitive drum, and the application time of a charging voltage, has thus been discussed to control the amount of exposure to make latent image potential settings constant (see Japanese Patent Application Laid-Open No. 2002-296853). According to such a method, the range between the maximum amount of light for forming an image portion potential (Vl) and the minimum amount of light for generating the non-image portion potential (Vd) can be changed based on the calculated thickness of the photosensitive drum to stably reproduce image density, line widths, and gradation.
According to the foregoing technique, a color image forming apparatus including a plurality of photosensitive drums can control the amounts of exposure of non-image portions on the respective photosensitive drums according to the thicknesses of the photosensitive drums. As a result, constant non-image portion potentials (Vd) can be obtained even if a common voltage value is applied to the charging rollers. In addition, the amounts of exposure of image portions for generating image portion potentials (Vl) can also be controlled based on the thicknesses of the photosensitive drums. As s a result, the charging voltages of the plurality of photosensitive drums and developing voltages applied to developing devices for developing electrostatic latent images on the respective photosensitive drums can be shared. The image forming apparatus can thus be reduced in size and cost.
However, an imaging forming apparatus that uses the DC charging system and performs non-image portion exposure (background exposure) on a surface-charged photosensitive drum has had the following problem. If the photosensitive drum is repeatedly subjected to a not-optimized amount of background exposure over a long period of use, the sensitivity of the photosensitive drum can vary greatly to cause an image defect such as a drop in image density. If the surface of the photosensitive drum is charged with a constant charging voltage, a primary charging potential increases because of a decrease in thickness associated with the use. To maintain a constant non-image portion voltage (Vd), the amount of background exposure is controlled to increase. In such a case, the amount of background exposure would become extremely large after a long period of use as compared to the initial use state of the photosensitive drum.
To suppress the sensitivity drop of the photosensitive drum due to optical fatigue, the amount of background exposure is desirably suppressed to a low level. For that purpose, the voltage applied to the charging device for charging the photosensitive drum is also desirably adjusted to be as low as possible. A color image forming apparatus including a plurality of photosensitive drums may include a plurality of charging devices for the respective photosensitive drums so that different charging voltages can be applied to the charging devices according to the thicknesses of the photosensitive drums. This can suppress the charging voltages for charging the respective photosensitive drums to a low level. In such a case, however, voltage circuits need to be prepared for the respective photosensitive drums. For example, a plurality of power supplies for applying voltages to the charging devices is needed. Accordingly, improvement is demanded in terms of miniaturization and cost reduction of the image forming apparatus.